Heat-sensitive copying-paper



c. s. MILLER 2,880,110

HEAT-SENSITIVE COPYING-PAPER March 31, 1959 Filed Deo. 2. 1954 United States Patent O c 2,880,110 c HEAT-SENSmvE COPYING-PAPER Carl S. Miller, St. Paul, Minn., assignor to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporation of Delaware Application December 2, 1954, Serial No. 472,556 12 Claims. (Cl. 117-36) This invention relates to heat-sensitive copying-paper suitable for thermographic reproduction of typewritten messages or other graphic subject-matter. More particularly, the invention relates to heat-sensitive copyingpaper having a backing member carrying a firmly bonded, thin, visibly heat-sensitive layer including color material distributed throughout, and visibly obscured by, fusible particulate material.

This .application is a continuation-in-part of my copending application Serial No. 747,338, led May 10, 1947, now U.S. Patent No. 2,740,896.

In the thermographic reproduction process here involved and for which my novel sheet material is specically designed, the heat-sensitive copying-paper is placed in contact with a surface of the original and the latter is then strongly and briefly irradiated with radiant energy which is preferentially absorbed by areas of the original forming the graphic message or subjectfmatter. The absorbed radiation is converted to heat energy in a pattern corresponding to the graphic `subject-matter, and the heat energy ows into the heat-sensitive layer and convertsthe corresponding areas thereof to a visibly distinct form. In the copying-paper of the present invention, a visible change results when the fusible organic particles melt Iand coalesce, thus permitting the initially obscured color material to become permanently visible, as will be further illustrated hereinafter. One typical application of thermographic reproduction as above described is in the copying of typewritten articles. The heat-sensitive copying-paper is held with the heat-sensitive surface in heat-conductive contact with the reverse side of the typewritten page, and suitable precautions are taken to prevent heat loss from the composite. The exposed printed surface of the original is then strongly and briefly irradiated. For small areas, suitable radiation is obtained from a standard 50G-Watt infra-red bulb with internal reflector, operated under overload conditions of 800 watts input and at a distance of about` three inches from the printed surface. The radiation is preferentially absorbed by the typewritten characters, vand the heat produced passes through the thinpaper to the heat-sensitive copying-paper where a visible reproduction is immediately formed. The operation has been termed back-printing.

`The copying of material printed on heavy paper or cardboard, or of pages printed on both surfaces, is more conveniently accomplished by front-printing. In this process a thin heat-sensitive copying-paper is used which is capable of readily transmittingthe radiationemployed without undergoing visible change. The copying-paper is placed against the printed surface but with the heatsensitive layer exposed, and the original is irradiated through the copying-paper. The heat produced on abs orption of the radiation in the printed characters passes through the thin backing to the heat-sensitive surface and resultsin the formation of a visible reproduction.

Reversing the copying-papenwill ineither` method result in areverse copy of the original when the copyingf" Patented Mar. 31, 1959 "ice c 2 paper is viewed from the side carrying the heat-sensitive coating.

For copying larger areas, such as typewritten pages, drawings, etc., it has been found convenient to employ a lamp system which focuses a narrow beam of the desired radiation across the page and then to scan the page with the beam. A typicallamp system for use in copying originals printed on the usual office letterheads` employs a lamp having a 200G-watt coiled tungsten ilament ten inches long, mounted in a reflector so as to produce a beam one-fourth inch wide. Scanning at a` rate of three inches per second with this lamp system has been found to produce excellent prints having good contrast and detail when using copying-paper capable of visible conversion at temperatures in the neighbor* hood of 65 C.

More intense radiation, or somewhat longer exposure time, or operation at higher ambient temperature, makes possible the utilization of copying-papers designed to operate at higher temperatures.

Electrically heated tungsten filaments provide an elfective and preferred source of radiation, but the maximum temperature conveniently obtained from such sources in making copies by the methods herein described is not much higher than about C. Most of the more desirable backings or sheet materials which I may employ are cellulosic, or at least organic in character, since highly heat-conductive b'ackings such as metal foil greatly reduce the clarity and definition of the reproduction. The organic backings, as also the paper of the original letter, drawing, or other article of which copies are most frequently desired, are subject to deterioration when heated excessively. c Consequently I prefer to select fusible materials which are capable of melting or fusing at temperatures not higher'than about 115 C., or in extreme cases not higher than about C. At the same time, it is necessary that the heat-sensitive coating remain unaffected during storage and shipment, and hence I find it necessary to employ fusible materials which will remain in the solid state at temperatures below about 60 C., or 'at least below about 50 C.

The heat-sensitive layer must be firmly attached to the backing member and must resist transfer in whole or in part to the original when pressed thereagainst during the copying operation. The presence of minor proportions of a binder material capable of forming strong self-supporting films when deposited from solution and which is non-fusing at the fusion temperature of the fusible particles provides effective bonding of the heatsensitive layer.

For best results in the reproduction of printed matter, and particularly of half-tone prints or the like which contain areas of line detail as well as massive darkened areas, certain further requirements as to timel and temperature of exposure or activation may be noted. The heat-sensitive composition must be capable of changing to a visibly distinct form having a desirably high contrast number with a minimum exposure and over a minimum range of temperature. Compositions which are converted instantaneously on heating to a specie temperature would be highly desirable for many purposes. However, somewhat slower-acting compositions which provide a contrast number of at least about 0.4 within less than one-half second, or preferably less than onetenth second, and which furthermore provide this entire degree of darkening within a temperature range of not more than about 20 C., and preferably not more than about 5 C., have been found to provide reproductions of good detail and contrast. In determining the contrast number the samples may conveniently be exposed, at specific temperatures and for specific time intervals as required, `by pressing them against heated metal' plates,

Typewriter ribbons and printing inks ordinarily contain carbon pigment which is highly absorptive of visible and infra-red radiation, whereas the paper on which such inks are printed. is relatively much less absorptive of such radiation; and originals printed with such inks are therefore preferably irradiated with such rays. Other radiations are useful, however, the significant requirements being that the radiation is preferentially absorbed byV those areas of the original which define the graphic subject-matter and that it is of sutcient intensity to provide the required heating effect when thus absorbed.

The following examples provide specific temperaturesensitive copying-papers which have been found to produce good results when employed in the reproducing of printed matter in accordance with the procedures described. However, it will be understood that the invention is not to be limited to these specific examples, since various equivalent compositions and backing materials may altematively be employed.

Example 1 Thin unsized flax paper was coated and impregnated with a mixture of finely divided dispersed particles of methylene blue and of mercuric stearate in a xylol solution of ethyl cellulose. Mercurio stearate is a waxy fusible organic material, insoluble in xylol; the particles fuse together to form a transparent film when heated to about 90-l00 C. and permitted to cool. On drying, the sheet was quite transparent, c g. when the sheet was held in contact with a printed page, the printing could be seen through the sheet; it had a faint blue color, and was unchanged on prolonged exposure to light. When used as a heat-sensitive copying-paper, it produced an intense blue facsimile copy of the graphic original. The sheet was most satisfactorily exposed with the coated surface in contact with the original printed surface, but usefully clear and exact copies could alternatively be prepared with the untreated surface in contact with such printed surface, or with the copying-paper suitably supported against the reverse side of the printed page. In this last-named modification, the thickness of the web supporting the heat-sensitive surface coating could be considerably increased.

Other equivalent particulate fusible materials, soluble strong film-forming binders, color particles, and volatile solvents may replace the specific materials here enumerated. In general, the amount of ethyl cellulose or other binder in such mixtures of binder and fusible particles may range from as low as about 5% up to about -20% or somewhat higher, but at much increased amounts the initial opacity of the coating is insufficient. This effect may be avoided by causing the binder component to be deposited as a partially or completely nontransparent, porous, self-sustaining stratum, as will now be disclosed.

ExampleZ A liquid coating composition is prepared from the following ingredients.

Parts by weight Hydrogenated fatty oil wax, M.P. 65 C. 1073.7 Nitrocellulose 382 Ethyl cellulose 62.4 Dioctyl phthalate 224.7 Acetone 5800 Toluol 2460 vmicroscopic particles and the suspension can be coated in a smooth, uniform, very thin layer.

There is then added a quantity of Diane blue` pig- `mehr to,l the. extent of4 aboutv 3-12% ,ofthewghto the 4, non-volatile portion of the suspension, and the pigment is thoroughly dispersed in the liquid. Diane blue is an infra-red-transmitting blue lake pigment.

The liquid coating composition is coated on thin clear transparent glassine and dried at room temperature. The product is light blue-gray in color, changing to a transparent blue-black when heated to about 65 C. Copies produced by the front-printing process, using this copying-paper and irradiating with radiation high in infra-red, have excellent contrastY and detail. The sheet is also useful for back-printing.

In the foregoing examples the color material is in the form of separate particles which are dispersed throughout the layer of fusible organic particles, the whole being held in place by a film-forming binder component which is infusible at the fusion temperature of the fusible particles. The next example illustrates a structure in which the color material is combined with the binder component. In both structures the particles of fusible material conceal or obscure the color material, the latter then becoming visible when the fusible particles are melted and converted to transparent nlm-form.

The fusible waxy component of the heat-sensitive coating of Example 2 may readily be extracted, without altering the structure of the non-fusingbinder stratum, by means of suitable selective solvents. When this is done, it is found that the binder remains in the form of a lightdiffusing and non-transparent porous but still strong and self-supporting web. Such a web may be locally permanently transparentized by impregnating it with a melted drop of the wax, which wets the porous binder layer, has substantially the same refractive index and is transparent in solid film form, and is compatible with the binder.

Example 3 A solution is prepared by dissolving 30 parts of ethyl cellulose, 15 parts of Rezyl 99" (an inert acetone-soluble non-drying semi-solid glycol phthalate type resinous plasticizer), and 0.3 part of Victoria Green WB soluble green dye in 255 parts of acetone. Separately there is prepared a dispersion of lead palmitate in acetone by ball-milling 20 parts of the insoluble waxy soap in 80 parts of the vehicle. Twenty part of the colored solution and 50 parts of the wax dispersion are blended together and coated in a thin uniform layer on a heavy smooth-surfaced backing member such as Kloth White paper, using a coating orifice of 6 mils. After drying at room temperature, the coated surface appears white, changing to green when heated to about 103 C. The sheet product is useful for back-printing.

In this example the soluble dye is retained by the binder which is colored thereby. The particles of lead palmitate obscure the colored binder in the'completed copying-paper.

Example 4 Methylene blue chloride, a blue dye, is dispersed in ethyl acetate to a concentration of 2% by ball milling. Mercuric stearate is similarly dispersed in ethyl acetate to a concentration of 20%. Three and one-half parts of the dye suspension and 25 parts of the wax suspension are mixed together, with addition of 4 parts of 12S-second nitrocellulose lacquer solution prepared as for conventional viscosity measurements. Coated on one mil ax tissue paper at a coating orifice of 5 mils and dried at room temperature, this composition produces a light blue copying-paper which converts to a brilliant dark blue color at about C. and is useful for both frontprinting and back-printing.

tion containing one part of w15 parts of Rezyl 99 resin, and 30 parts of ethyl cellu- In Figure 1, a supporting base 10 is coated on one Dispersion A:'

` A mixture of 50 parts dispersion B, 5 parts dispersion A, and parts solution A is coated on plain transparent cellophane at a coating orifice of 4 mils, and the product is dried at room temperature, producing a light blue sheet which changes to dark blue when heated to approximately 85 C. and is useful for both front-printing and back-printing.

Example 6 Lead palmitate is dispersed in acetone to a concenltration of 20% by ball milling, and 20 parts of the dispersion ismixed with l0 parts of a colored binder solu- Bismarck Brown RV Base,

lose, all dissolved in 225 parts of acetone. Whitepaper coated with the mixture at a coating orifice of 3 mils, and dried, provides a copying-paper which changes from light tan to orange-brown at approximately 95 C. and is useful for back-printing.

The invention is further illustrated by the accompanying drawing, in which Figures 1 and 2 are diagrammatic, enlarged representations in cross-section of typical examples of my novel heat-sensitive copying-paper.

surface with a heat-sensitive coating 11 containing color particles 12 dispersed therethrough and concealedl or lobscured by light-diffusing fusible organic particles 13, the coating being held together and to the base 10 by a binder 14 which is non-fusing at the fusion temperature of the fusible particles 13. The fusible material has good wetting properties toward the binder, and the two are preferably of substantially the same refractive index and mutually compatible.

The heat-sensitive copying-paper of Figure 2 likewise has a backing member or supporting base 20 and a visibly heat-sensitive coating 21; in addition, the sheet is provided with a thin protective surface coating 25, and at least one of said base 20 and coating 24 must be transparent. The visibly heat-sensitive coating in the structure of Figure 2 consists of fusible particles 23 which are held together and to the base 20 by a colored binder component 24, the binder component being concealed or obscured by the fusible particles.

The color particles of the structure illustrated in Figure 1 may be completely inert toward the fusible particles and the binder component as in Example 2, in which case the color particles themselves become visible when the fusible material is converted from the opaque particulate to the transparent continuous state. The color particles of Figure 1 may alternatively be partly or completely soluble in the molten fusible material as in Example l, or may otherwise cause the development of color in the molten fusible material; in such case the particles are reduced in size, or disappear as such, during the copying process, while a visibly distinct appearance is produced at the heated areas. Figure 2 illustrates a structure in which the color material is initially combined with the film-forming non-fusing binder component; the preparation of such a structure 1s described in Example 3. Still other structures and combinations may be provided in which the fusion of the concealing or obscuring mass of fusible organic particles makes visible a color material contained within said mass of particles.

The thin surface layer of film-forming binder material illustrated in the exemplary structure of Figure 2 further protects the heat-sensitive layer from abrasion or other mechanical attack. In order to avoid solution of the :fusible particles or binder component and transparentization of the heat-sensitive coating during application of such layer, the volatile vehicle employed in forming such protective surface coatings is selected from those liquids which are non-solvents for the components of the heatsensitive layer. Suitable film-forming resins or polymers which are soluble in heptane or dispersible in aqueous vehicles, for example, are applicable in forming useful thin protective layers on the heat-sensitive sheet material described in Example 2.

Where the copying-paper employs a transparent backing such as cellophane or glassine, the protective surface coating may be made non-transparent, e. g. by incorporation of a pigment or dye. With such a copying-paper, an infra-red-transmitting colored protective surface coating permits the sheet to be used for either front-printing or back-printing. Cpaque backings, on the other hand, require that any protective surface coating be transparent so that the visible change occurring in the heat-sensitive layer may be viewed through such coating.

The waxy fusible metallic soaps of Examples l and 3-6 soluble metal salt. The precipitate is recovered, washed,

dried, and then suspended in the form of line particles in the desired volatile vehicle by ball-milling or other suitable means. The hydrogenated fatty oil wax of Example 2, or hydrogenated castor oil wax melting at about C., or other waxes or waxy materials as well as other organic fusible materials melting within the preferred general range of about 60l15 C. are also useful, as well as` mixtures of such materials.

The fusible material in particulate form serves to mask or obscure from view mostor all of the color material associated therewith. On heating the coating to or above the fusion temperature, the fusible particles melt or fuse and the color material becomes visible, either through the transparent fused and solidified iilm of fusible material or as a solution in such film or in some other manner. Hence the fusible material must be normally transparent, stable against actinic light or atmospheric humidity experienced during storage and use, and capable of melting to a liquid without appreciable volatilization or decomposition at available and useful temperatures. A number of fusible materials meeting these requirements have been set forth in the accompanying illustrative exemples, and others will be apparent in view of the above disclosures. Where slow recrystallization of the consolidated fusible material might subsequently cause opacifying of such layer, improved permanency of copy is attained by using mutually compatible fusible and binder materials or by incorporating small amounts of plasticizers or other modifiers selected to improve the compatibility therebetween.

What I claim is as follows:

1. A heat-sensitive copying-paper adapted, on being placed in heat-conductive contact with an original having preferentially radiation-absorbing areas and on irradiation of said original as hereindescribed, to provide a visible reproduction of said original, said copying-paper comprising a backing member carrying a firmly bonded, thin opaque, visibly heat-sensitive layer comprising color material uniformly interspersed within an obscuring mass of particles of normally transparent stable organic fusible solid melting to a liquid without appreciable volatilization -or decomposition at a temperature within the range of comprsing ra backing member carrying a thin, opaque, visibly heat-sensitive layer comprising color material unimember by a film-forming binder which is non-*fusing at the melting temperature of said fusible particles.

3. The heat-sensitive copying paper of claim 2 in which the color material is lpresent in the form of separate particles.

`4. The heat-sensitive copying-paper of clairn 3 in which lthe color material is an inert pigment.

5. Theheat-sensitive copying-paper of claim 3 in which the color material is a soluble dye.

6. The heat-'sensitive copying-paper of claim 2in which the binder serves as the color material.

7. The heat-sensitive copying-paper of claim 6 in which the binder is colored with a soluble dye.

8. As a new article of manufacture suitable for use in the copying of printed matter by a rapid direct process involving only the application of intense radiant energy as herein described, a thin sheet mater'al comprising a Ysupport member carrying a rmly bonded, thin, opaque coating comprising color material uniformly interspersed within an obscuring mass of particles of a normally transparent waxy material melting at atemperature within the range of about 50-l50 C.

9. As a new article of manufacture suitable for use in the copying of printed matter by `a rapid direct process involving only the application of intense vradiant energy as `herein described, a thin sheet material comprising a support member'carryng a thin opaque coating comprising color material uniformly interspersed within an obvscuring'mass ofparticles of a normally transparent waxy material 4melting at a temperature within the range of yabout 50150 C., said coating being bonded together and to said support member by a film-forming binder which is non-fusing at the melting temperature of said waxy material.

10. The article of claim 9 lin which the color material is asoluble dye.

11. The .article of claim 10 in which .the color material is dissolved in the film-forming binder.

12. A heat-sensitive copying-paper adapted, on being placed in heat-conductive contact with an original having ,preferentially radiationfabsorbing areas and on irradiation of said original as herein described, to ,provide a visible reproduction of said original, said copying-paper comprising in order a backing member, a rmly bonded, thin, opaque, visibly heat-sensitive layer, and a protective .surface coating at least one of said backing member and said surface coating being transparent; said heat sensitive layer comprising color material uniformly interspersed'within an obscuring mass of particles of normally vtransparent 'stable organic fusible solid melting to a liquid vwithout vappreciable volatilazation or decomposition at a temperature within the range of about 50-150 C.

References Cited in the tile of this patent :UNITED STATES .PATENTS `2,'188,`590 Bjorksten Jan. 30, 1940 '2,269,038 Perry Jan. 6, 1942 V2,306,625 Cummings Dec. 29, 1942 2,519,660 VJames Aug. 22, 1950 12,668,126 ITaylor Feb. I2, 1.954 

1. A HEAT-SENSITIVE COPYING-PAPER, ON BEING PLACED IN HEAT-CONDUCTIVE CONTACT WITH AN ORIGINAL HAVING PREFERENTIALLY RADIATION-ABSORBING AREAS AND ON IRRADIATION OF SAID ORIGINAL AS HEREINDESCRIBED, TO PROVIDE A VISIBLE REPRODUCTION OF SAID ORIGINAL, SAID COPYING-PAPER COMPRISING A BACKING MEMBER CARRYING A FIRMLY BONDED, THIN OPAQUE, VISIBLY HEAT-SENSITIVE LAYER COMPRISING COLOR MATERIAL UNIFORMLY INTERSPERSED WITHIN AN OBSCURING MASS OF PARTICLES OF NORMALLY TRANSPARENT STABLE ORGANIC FUSIBLE SOLID MELTING TO A LIQUID WITHOUT APPRECIABLE VOLATILIZATION OR DECOMPOSITION AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 50-150*C. 